Muszę nawozić roślinę doniczkową.

Muszę means I must or I have to.

It is the 1st person singular form of the verb musieć (to have to / must).

• musieć = to have to
• muszę = I have to

So the sentence is talking about my obligation.

Because in Polish, the verb ending already tells you who the subject is.

Muszę already means I have to, so ja is usually unnecessary.

• Muszę nawozić roślinę doniczkową. = neutral, natural
• Ja muszę nawozić roślinę doniczkową. = more emphatic, like I have to fertilize it

Polish often drops subject pronouns unless they are needed for contrast or emphasis.

Nawozić means to fertilize or to feed with fertilizer.

It is used when you give a plant nutrients, for example with liquid fertilizer or plant food.

It does not mean:

• podlewać = to water
• sadzić = to plant
• przesadzać = to repot

So nawozić roślinę is specifically about giving the plant fertilizer.

Because after muszę, Polish normally uses an infinitive.

This works very much like English:

• I must fertilize
• Muszę nawozić

Other similar examples:

• Muszę pracować. = I have to work.
• Muszę iść. = I have to go.
• Muszę kupić nawóz. = I have to buy fertilizer.

So the pattern is:

muszę + infinitive

Because the plant is the direct object of the verb nawozić, so it has to be in the accusative case.

The dictionary form is:

• roślina doniczkowa = a potted plant / houseplant

But in this sentence, it changes to:

• roślinę doniczkową

That is the accusative singular feminine form.

So:

• roślina → roślinę
• doniczkowa → doniczkową

This is one of the most common case changes in Polish.

Because the adjective has to match the noun.

In Polish, adjectives agree with nouns in:

• gender
• number
• case

Here, roślina is:

• feminine
• singular
• accusative

So the adjective doniczkowa must also become feminine singular accusative:

• roślina doniczkowa → nominative
• roślinę doniczkową → accusative

This agreement is a basic feature of Polish grammar.

Polish does not use articles like English a and the.

So roślinę doniczkową can mean:

• a potted plant
• the potted plant

The exact meaning depends on context.

If you want to make it more specific, Polish can use words like:

• tę roślinę doniczkową = this potted plant
• jakąś roślinę doniczkową = some potted plant

But very often Polish simply leaves that information unstated.

Usually, yes, it can be translated as houseplant in many contexts.

Literally, though, it means potted plant:

• roślina = plant
• doniczkowa = pot-related / in a pot

So it refers to a plant grown in a pot. In practice, that often overlaps with houseplant, but potted plant is the more literal meaning.

Because nawozić is imperfective, it often suggests:

• a general activity
• repeated care
• an ongoing task
• something you need to do as part of plant care

So Muszę nawozić roślinę doniczkową can sound like:

• I need to fertilize my potted plant
• I have to keep fertilizing the potted plant
• I need to fertilize the houseplant as part of normal care

Context decides exactly how specific it is.

If Polish wants to emphasize a single completed action, it usually uses a perfective verb instead.

Yes. Polish word order is more flexible than English because the endings show the grammatical roles.

The sentence:

Muszę nawozić roślinę doniczkową.

is a natural, neutral order.

But you could also say:

• Roślinę doniczkową muszę nawozić.
  This puts more emphasis on the potted plant.

• Nawozić muszę roślinę doniczkową.
  This is possible, but less neutral and more marked.

So the original version is probably the best one for a learner to use first.

Polish stress is usually on the second-to-last syllable.

So here you get:

• MUszę
• naWOzić
• roŚLInę
• doniczKOwą

That regular stress pattern is one reason Polish pronunciation becomes easier once you learn the syllables.